I've been conducting a little research with the help of a good friend and here are the findings:

Lots that have been built and packaged using the game update will not install into an unpatched base game.  You will get an error message that an update is required before the lot can be installed.  This is a good thing really as the lot won't install it won't case any problems with your game.

Lots built and packaged with WA installed can be installed into a patched base game and they work just fine.  However, any items used that are specific to WA (roof recolours, plants/trees - build/buy items) will be replaced with base game items.  Things like fence and stair recolours will make the transition just fine.

Just in case you're not sure of which patch/update I'm referring too it's the game update 1.5/1.6.  I'd also strongly recommend the game update 1.7/2.2 also to avoid any game save issues caused by the previous patch.

So yay!  WA lots in the base game! woohoo!  Kudos to EA and a huge hug to Debra for testing this for me!